Soils of the Main Ethiopian Rift Valley escarpment: A transect study

Abstract The mountainous environment of the Ethiopian highlands has a great variety of ecotopes and thus demands great flexibility in land management. Different climatic conditions and landscape positions induce different soil forming processes, leading to various soil types with specific risks and potentials. The present study portrays a soil sequence of the central section of the Main Ethiopian Rift Valley, from the footslopes of the eastern escarpment to the marginal volcano structures. Six profiles under natural vegetation are described for classification according to the USDA Soil Taxonomy and World Reference Base for Soil Resources classification systems, and special site characteristics are discussed. The Acacia savannah of the footslopes (1900 m a. s. l.) is characterized by Vertisols (Mazic Vertisols/Aridic Haplusterts), with often pronounced effects of seasonal waterlogging. All other soils are well-drained and reflect the general increase in rainfall with elevation and slope, causing a decline in topsoil pH and a change from cation-rich clay soils of the Podocarpus-dominated forest at 2300 m a. s. l. (Mollic Nitisols/Typic Palehumults) to strongly-leached Humic Umbrisols/Humic Dystrudepts of the Hagenia-dominated forest around 2600 m a. s. l. The highland savannah plain (2700 m a. s. l.) with a drier and cooler environment has typically Mollic Cambisols/Dystric Haplustepts, which are less leached and have a rather brownish colour. At 2900 m a. s. l., Niti-umbric Alisols/Andic Hapludalfs are found in the Hypericum forest at the midslopes of the marginal volcanoes receiving high rainfall, whereas soil development is at a more initial state in the Erica-dominated forest at 3200 m a. s. l. (Umbric Andosols/Alic Hapludands). Clay mineral composition is kaolinite-dominated for the upper five profiles, with a high proportion of poorly crystalline components in the upper savannah and the volcano upslopes. The lowermost profile probably has a polygenetic origin indicated by an abrupt change from a smectitic to a kaolinitic composition in the subsoil. Soil development on quite homogeneous bedrock appears essentially controlled by relief and climate, underlining the suitability of the region as a model area for in-depth gradient studies on ecosystem processes and land use.

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